fix 3 issues reported by xigao in regression.py

some problem was introduced in pull:  https://github.com/autotest/virt-test/pull/1516

### @xigao 's comments

```
According to the patch, here is the regression results without highlight.
http://kvm-perf.englab.nay.redhat.com/results/request/highlight-T-test/version7/fio.html
Based on above the link, here is some small faults:
1. Block_size/Iodepth/Threads is valued by float.
2. "%Diff between Avg" value keep origin data and leave many decimal points.
3. Positive "Significance" value is removed "+" signal.
```

The last patch added test data to report (requested by @ronenhod)